Not Applicable.
Not Applicable.
The present invention relates to medical devices that are implanted in a body and receive their operating energy from an external source, e.g., a transmitter, without transdermal circuit conductors. It particularly relates to nerve stimulation or conditioning devices in which such external energy is received by an implanted receiving coil and applied to an implanted electrode. Such systems are shown in U.S. Pat. Nos. 5,094,242; 5,211,175 and elsewhere.
The device described in the aforesaid patents is shaped like a thumb tack and has a needle-like electrode structure protruding from a disk-like cap. The cap contains a coil which operates as an energy-receiving antenna, and the coil is inductively energized by an external transmitter that applies an rf frequency signal to a transmitting antenna located proximate to the implanted receiver and outside of the body. In that device, the external transmitter transmits at a frequency to which the internal coil is tuned, to enhance the efficiency of energy coupling through the skin and into the neural stimulation electrode. The two coils effectively act as windings of a transformer coupled by a magnetic field that is created by rf current flow in the external coil.
The nature of this coupling is relatively inefficient, and depends very much on the spacing, position and shape of the respective coils. In general, the magnetic field created by a transmitting coil, and the coupling efficiency between two such coils, requires them to be closely spaced and well aligned or contiguous. It has been suggested in U.S. Pat. No. 5,891,183 that one or both of these coils may employ non-circular windings to enhance the magnetic coupling coefficient between the two coils and make the energy coupling efficiency less sensitive to their relative positioning. For a subcutaneous device such as the device illustrated in U.S. Pat. No. 5,211,175, the receiving coil may reside in a relatively well-defined position at a shallow depth below the skin and it is therefore possible to position the external transmitter fairly close to and roughly in alignment with the receiver. However, coupling efficiency remains a serious factor. If the contemplated neural stimulation regimen requires continuous, repeated or relatively long-term application of electrical stimulation, and battery power is contemplated for the external transmitter, overall efficiency of electrical usage as well as stability of the power source both become important concerns.
It would therefore be desirable to provide an external transmitter assembly that is power efficient.
It would further be desirable to provide an external transmitter assembly that uniformly and dependably couples modulated power to a passive implanted receiver.
One or more of the foregoing or other desirable ends are achieved in accordance with the present invention by an external transmitter assembly for powering an implanted medical device, wherein a battery drives first and second power supplies that energize a modulation circuit and an amplifier, respectively, to drive an rf coupling antenna. The second power supply is output limited and couples through holdup capacitors to the amplifier, allowing momentary high current operation without affecting rf or pulse modulation portions of the circuit. The transmitting antenna has a diameter greater than that of the implanted receiving antenna, and is adhered to the skin of a patient over the implanted device. The transmitting coil is preferably tuned with a small variable circuit element to resonate at the resonant frequency of the implanted receiver to more effectively couple energy between the two coils. A local oscillator operating at high frequency is divided down to form clock signals for pulse generation, and three distinct high frequency signals provide switching frequencies for the power supplies and a precisely controlled rf transmitter operating frequency. A field programmable gate array (FPGA) powered by the first power supply controls pulse shape and timing regimens for a defined neurologic treatment. By providing the two power supplies with high switching frequencies that are different from each other and from the main transmitter operating frequency, aliasing and power stealing effects between the units are avoided. In addition, an auto power-down circuit connects the battery to the power supplies. That circuit connects battery power upon user actuation of a main power switch, and disconnects the battery when the signal state of an FPGA signal enable line indicates a treatment cycle is over. The auto power-down circuit draws little power, thus assuring that battery life is unimpaired if the user forgets to turn off the transmitter, or leaves it in a shut down state for extended periods of time.